Hydrocarbon wells typically produce a percentage of water along with the hydrocarbon fluids, which may be oil or gas. A variety of separators exist for separating the water from the hydrocarbon, operating on principles such as gravity separation, centrifugal separation, and vortex separation.
A more recent type of separation unit, called a coalescing filter, comprises a plurality of tubes mounted in a cylinder parallel to the flow. The tubes are electrically conductive and insulated from the plate. The operator supplies a voltage to the tubes. Dipolar water droplets contained in the oil phase are oriented by an electrostatic field from the voltage in the tubes in a way that makes them collide or coalesce with each other. This effect causes water within the well fluid to form into larger water droplets. Larger water droplets generally move and separate faster than smaller droplets, reducing the retention time to remove water from oil in gravity separation. The well fluid normally flows from the coalescence unit to a second stage for more separation.
A second stage could be another coalescence unit or it could be a unit of a different type, such as a gravity unit, a centrifugal unit, a vortex unit, or a dielectrophoresis unit. A dielectrophoresis unit also uses a high voltage electrostatic field, however the field is configured to force the water droplets into designated sections of the unit and thereby form streams of water. Electrodes in the form of undulating sheets are used. The electrode sheets are closely spaced and arranged side-by-side to define constrictive passage portions and widened passage portions. The passages formed by the sheets force the water droplets to move towards the stronger section of the electrostatic field, which is at the narrower portions. The forces imposed by the electrostatic field guide the water droplets into these predetermined passage portions, where they form high water content sections of liquid that will easily separate immediately downstream of the unit.
Regardless of the type, typically, the separators are part of a processing system located near the well site. In the case of subsea wells, the operator normally places the processing equipment on a platform, which may be either fixed leg or floating. In a few instances, separators have been placed in a subsea location, which may reduce the distance that the well fluid has to be pumped, and the distance that the separated water has to be pumped before being injected into a water injection well. Coalescing filters can be sized small enough to be placed within subsea flow lines to enhance subsea well fluid processing.
One problem that may arise, however, with a subsea processing system deals with cleaning the flow lines. Well fluids often contain substances, such as paraffin, that coat the interior of the flow lines. A variety of processes exist for removing the coating, including the use of chemicals and mechanical cleaning. In mechanical cleaning, the operator may utilize a pipeline pig, which is a cylindrical member that fits sealingly within the flow line. The body of the pig has scraper elements to scrape and remove the coating. The operator applies fluid pressure to the opposite end of the pig to cause it to flow through the flow line to perform the cleaning operation.
A separation unit typically would not be configured to accept a pig, particularly a coalescence unit because it comprises a large number of small diameter tubes mounted parallel to the flow. For a subsea separation unit of this type, a bypass line with control valves would need to be provided. Constructing a bypass line would add to the expense and maintenance of a subsea processing installation.